1. Field of the Invention
The present invention relates to a tool that advances in a spiral manner to make a through hole, a recess or a threaded hole in a workpiece, more particularly to an improved cutting tool that, even when damaged or broken, can still be utilized.
2. Description of the Prior Art
A conventional cutting tool used to make a through hole or a recessed hole (whether it is threaded), regardless of whether it is for manual operation for use with a mechanical device, generally includes an elongated rod of a certain length and is formed in sequence along an axial direction thereof with a working portion at one end having teeth or blades for cutting workpieces, a clamp portion at the other end for holding by the user with an appropriate tool so as to turn along a predetermined direction, and an extension connected between the working portion and the clamp portion.
Take the ordinary tap as an example. In use, since the tap has to be disposed in a hole having an internal diameter smaller than its external diameter, and has to be turned back and forth to thereby cut the inner wall of the hole and to remove scraps. In addition, to facilitate operation, the distal end of the working portion is configured to be a taper having a gradually reduced external diameter. During the process of the user turning the tap back and forth, the tap may easily break, especially at the distal end of the working portion, and the broken portion is usually located in the hole and tightly stuck in the hole that is being processed. This is a big problem for the user since the making of the thread hole is not yet finished and the broken tap stuck in the hole is difficult to remove.
In an attempt to solve the above-mentioned problem, U.S. Pat. No. 5,299,105-discloses the pouring of a lubricant into a threaded hole to facilitate removal of a broken tap from the threaded hole. However, since lubricant cannot penetrate into the threads, the effect is not satisfactory. U.S. Pat. No. 5,934,385 teaches the use of a pressurized fluid to control or drive the tap. However, the construction is complicated and costly, and is therefore not widely adopted.
A primary object of the present invention is to provide a new cutting tool that allows easy removal of a broken part from the workpiece.
Another object of the present invention is to provide a cutting tool that, although broken, has a part remaining on the surface of the workpiece to allow the user to operate to thereby remove the broken part.
A further object of the present invention is to provide a cutting tool that prevents the most possible breakage from occurring at the working portion.
Still another object of the present invention is to provide cutting tool that when a part other than the working portion thereof is broken, has the remaining parts connected to the working portion to allow the user to continue the cutting operation.
In order to achieve the above-mentioned objects, the cutting tool according to the present invention includes a working portion adapted to cut a threaded hole or a hole in a predetermined workpiece, a first clamp portion provided at an end opposite to the working portion and adapted to be clamped by a clamping device to facilitate local application of force, and a weak portion disposed between the working portion and the first clamp portion and adapted to bear a torsional force smaller than that borne by other portions. The weak portion, during use of the cutting tool in a cutting operation, is always located on the surface of the workpiece and will break when the cutting tool cannot stand an excessive torsion. The broken portion will likewise always be located on the surface of the workpiece, whereby the user can utilize the second clamp portion to continue with the processing operation or to remove the broken portion from the workpiece.